This invention relates to a skin preparation device that can be used to penetrate the outermost layer of skin to allow for the penetration and/or displacement of the stratum corneum. A typical use of such invention is in the non-invasive monitoring of electrophysiological signals. This skin preparation device may be integrated into the electrodes of a biopotential sensor.
The human skin tissue is composed primarily of connective tissue, the dermis, covered by a protective layer, the epidermis. The outermost layer of the epidermis is the stratum corneum. In the study of electrophysiological monitoring using surface electrodes, the stratum corneum is significant, due to its function as a protective barrier. The stratum corneum is on average approximately 20 μm thick, and is composed primarily of denucleated, dead skin cells that are inherently a source of high electrical impedance. Very low amplitude signals are associated with some electrophysiological recordings, particularly electroencephalography (EEG); therefore, it is important to optimize the signal acquisition and minimize noise artifact. Thus, impedance measured at the interface between a patient's skin and the electrode used for acquiring the electrophysiological signals is an important consideration in biopotential monitoring. Additionally, variance in impedance between electrode sites can result in unwanted noise in the signal. Good electrical conduction between the patient's skin and the electrode can be better achieved by removing or penetrating the stratum corneum layer of the epidermis.
The most common electrodes that are applied to the surface of the skin often require that the skin be prepared before the electrode is applied. Preparation of the skin typically begins with cleaning off the surface with alcohol to remove dirt and oils, followed by abrasion of the skin's surface with an abrasive material or a grit-impregnated gel to remove the stratum corneum layer. To simplify the skin preparation process that is required prior to application of an electrode, several forms of self-prepping electrodes have been developed. Such electrodes, which have an integrated mechanism for achieving this preparation of the skin, provide numerous advantages over standard EEG or electrocardiogram (ECG) electrodes because they eliminate the need for the additional step of skin preparation as well as the need to have the abrasive material available in a clinical setting.
In many of these developments, the means by which the skin preparation occurs is by employing a textured component in conjunction with the electrode, which when pushed against the skin attempts to abrade or penetrate the outermost layer. This textured component may be a part of the electrode surface itself or an independent part affixed to the electrode surface. International Patent Application WO 02/00096A2 describes a means of collecting EEG using a “volcano tip” tine structure, in which tines formed from perforations in the electrode material are used to abrade the outer layers of skin and improve electrical contact. This application does not describe the design or manufacturing procedures for the volcano tip tine structure.
U.S. Pat. No. 5,305,746 issued to Fendrock et al. describes an electrode which provides a textured component by way of an integrated array of non-conductive flexile tines. The flexile tines are of length 0.025″-0.110″ and of thickness 0.002″-0.015″, and are embedded in a wet conductive gel. The flexile tines part the stratum corneum layer to expose the low impedance layers without scratching or abrading deeper layers of the skin. Although this device does generally reduce the measured impedance at the skin to electrode (skin to gel) interface, the mechanism by which flexile tines part the skin varies from person to person and skin type to skin type. Long flexile tines lack uniformity of orientation and insertion angle into the skin. Rigid tines may provide better control over the tine orientation and the mechanism by which they bypass the stratum corneum. Due to their size, macro-sized tines, as described in U.S. Pat. No. 5,305,746, limit the potential density of the tines in the array thereby also limiting the ability to reduce the overall electrode size and overall area of skin that is affected while maintaining equivalent signal quality. In addition, long tines facilitate being pressed too deeply into the skin causing unnecessary penetration beyond the stratum corneum while a shorter tine limits the deformation of the skin. Consequently, the advantage of an array of shorter and more rigid tines is that they can produce more repeatable low impedance signals with potentially less irritation of the skin.
U.S. Pat. No. 5,309,909 issued to Gadsby discloses a skin preparation and monitoring electrode that penetrates a patient's skin prior to acquiring biopotentials. The electrode has tines, mounted on the concave surface of a dome, that penetrate first the conductive layer of the electrode and then the skin when force is applied to the dome causing it to deflect towards the skin. Upon cessation of the application of force, the tines retract with the movement of the dome. The complexity of this design does not support the cost effectiveness and ease of manufacturing required for a disposable electrode and skin preparation device.
The concept of using an array of shorter rigid tine structures for skin penetration is commonly associated with the applications of biopotential signal acquisition and transdermal drug delivery mechanisms. Among rigid tine array designs presented in journal and patent literature, there is a variety of tine array structures, materials, and dimensions, all optimized for their particular application.
International Patent Applications WO 2004009172A1, WO 2007075614A1, WO 2007081430A2 describe microneedle devices for delivering a drug to a patient via the skin. These needles typically have a channel through the middle allowing fluid to pass through the microneedle array or they are coated with a drug, or active component that is intended to dissolve in the skin beneath the stratum corneum. These needles for transdermal drug delivery have no conductive requirement because they do not serve to transmit any electrical signal away from the skin. These microneedles utilize lithographic processes on silicon in order to be created at such small scale.
An example of an additional application of an array of spikes used to achieve electrical contact with a series of very closely located electrode sites is described in U.S. Pat. No. 7,103,398 B2, issued to Sieburg. In this patent Sieburg describes a device for sensing electrical signals on the surface of human or animal skin. The device is comprised of a substrate containing a plurality of electrodes with each of those electrodes having one pointed contact end facing away from the substrate. In this design, each pointed contact, or “tine” is coupled electrically to an independent electrode site, rather than having multiple tines together penetrate an area of skin from which the signal will be conducted to the electrode surface.
U.S. Pat. No. 6,622,035, issued to Merilainen et al. also aims to effectively acquire biopotential signals with an electrode comprising an array of cylindrical or tapered “spikes” to make the skin more permeable. Each electrode is described as having 100-10,000 (ideally 400-2000) spikes per electrode, with the length of the spikes ranging from 50-250 μm (≦0.010″) from the carrier or electrode surface. A subsequent patent, U.S. Pat. No. 6,961,603, also issued to Merilainen describes the same spike geometry and spike density; however such patent also teaches injection molding the “spikes” using a non-conductive material which will then be coated with a conductive layer such as silver-silver chloride. Such “spike” arrays, in addition to being conductive, are very small in size, fine in geometric characteristics and high in number spikes per array. These factors result in a non-cost effective design for molding, particularly for a disposable device.
U.S. Pat. No. 6,690,959, issued to Thompson also teaches the use of “nano-spikes” to penetrate the epidermis of the skin for collecting electrical biopotentials. The spikes are formed using a Microelectromechanical System (MEMs) construction technique and are subsequently coated with a conductive metal. Besides an indication that the nano-spikes are 10 μm in length and have an angularly disposed end shaped to assist in penetration of the cornified layer of the skin, no further detail regarding the geometry of the spikes is offered.
Similar biopotential signal acquiring devices have been created using carbon nanotubes. This approach is a highly effective means of creating very small, conductive tines in an array, however the cost and time associated with the growth of these arrays is currently prohibitive for integration into a high volume disposable product. The same drawbacks apply to microneedles formed using dry-etching of silicon, as it is a multi-step manufacturing process with high development costs.
One object of the proposed invention is the transduction of low impedance electrophysiological signals using a device that employs an array of sharp, rigid structures that can be integrated into a set of one of more electrodes to conduct the signals to a monitoring system. A further object is to provide a device that can be mass produced, for this application, at a cost appropriate for a disposable use product.